Various multicast packet switches have been proposed in the past, for example those by J. S. Turner (IEEE Transactions on Communications, 36, no. 6, June 1988, pp.734-743) and T. T. Lee (IEEE Journal on Selected Areas in Communications, 6, no. 9, December 1988, pp.1455-1467). Such switches are normally modular in design and are intended for implementation on a large scale, coupled for example in a delta network, so as to enable large numbers of inputs/outputs to be handled.
A problem with such multicast fabrics is how to handle contention at nodes within the fabric, where collisions can occur between cells (packets) contending for the same paths within the fabric. Blocking arises when a cell is unsuccessful in gaining control of a suitable path through the fabric. A means must be provided for enabling the cell to retry transmission so as to complete its passage through the switches. Lee (ibid) describes a particular form of non-blocking multicast circuit, whereas Turner (ibid) handles blocking by providing buffers for the cells at the nodes within the multicast fabric and which hold the cells until a successful retry in transmission through further nodes in the fabric can be made.
The present invention is concerned with switches for multicast fabrics which handle cell replication and blocking in a novel way. They can be produced cheaply in small gate arrays, and therefore are suitable for production in modular form to enable multi-input/output packet switches to be produced.